The classical and devastating dry eye states historically are xerophthalmia and keratomalacia. These grave conditions are caused by dietary deficiencies especially lack of protein and avitaminosis A and invariably lead to blindness. Fortunately, these grave diseases are rare in the technologically developed countries including the United States but still are fairly common in the third world countries.
Unfortunately, the population of the developed countries is not free of the "drying of the eye". It has been recognized only in the last decade that many of the eye complaints related to persistent irritation, burning sensation, conjunctival vasodilation, inflammatory reaction, occasional excessive tearing, and photophobia are usually caused by a dry eye state. Recent progress in tear film physiology enabled us to elucidate the underlying causes of such nonspecific and often marginal dry eye conditions which can be traced to certain tear film abnormalities resulting in pathological instability of the tear film over the corneal surface. Such tear film instability can and usually does cause pathological changes in the underlying epithelium. The resulting epitheliopathy further diminishes tear film stability, which in turn degrades the epithelial surface even more. Primary ocular surface disorder can also cause tear film instability which then enhances the gravity of the surface disorder.
The mainstay of the treatment of the dry eye conditions since antiquity has been the topical instillation of lubricating and wetting eye drops, tear substitutes or artificial tears. It is, however, important to realize that the tear secretion rate is pathologically diminished only in a relatively small fraction of the patients suffering from some dry eye state. Often stable tear film cannot be formed over the ocular surface despite an adequate amount of tears. Hence, the artificial tear formulations should not only supplement deficient fluid but should be capable of increasing tear film stability by forming stable fluid films over the ocular surface. These formulations should also have a beneficial effect on existing surface epitheliopathy, since such a pathology adversely affects fluid film stability.